1. Field of the Invention
This invention relates to an optical system for a microscope that can direct radiant energy to and receive radiant energy from a surface at grazing angles.
2. Description of related Art
Many industrial and scientific applications, particularly spectroscopic applications, require illuminating a surface by focusing a beam of radiant energy onto the surface at grazing angles of incidence. The term grazing angles, also referred to as glancing angles, corresponds to angles between 60.degree. and 90.degree. as measured from a line that is normal to the surface. Many applications also require observing energy reflected from the surface at grazing angles of reflection, and often a surface must be simultaneously illuminated and observed at grazing angles.
One method for observing a surface grazing angle of incidence and reflection is to geometrically align a beam of incident energy and a detector with the surface so that the energy is incident to the surface at grazing angles of incidence and reflects from the surface to the detector at grazing angles of reflection. Geometrically positioning a source, receiver and surface, however, is cumbersome and often totally impractical.
Another method for obtaining grazing angles of incidence and reflection in an optical system having a practical size involves using a multiple mirror configuration to curve an incident beam of energy to a surface at grazing angles by sequentially reflecting the beam of energy with a series of mirrors at a high angle of incidence relative to each mirror. The multiple mirror configuration, however, cannot form a sharp image on the surface because a part of the beam of energy that travels to the surface from a remote point must start nearer to the mirrors and travel a greater distance to reach the surface. The resulting difference in effective focal lengths for different areas of the beam of energy necessarily increases the minimum area needed to form a focus on the surface at grazing angles and, therefore, distorts the image at the surface. A microscope that uses a multiple mirror configuration cannot obtain high magnifications because of the poor image quality resulting from the poor focus on the surface.
U.S. Pat. No. 2,198,014 discloses a magnifying apparatus comprising a small convex reflector imbedded within a concave mirror. The mirror arrangement may form an image of a sample that is illuminated with energy transmitted through the sample at an angle relative to a normal to the sample. The magnifying apparatus appears to collect light over a large field of view from a small area on the sample or to collect light from a large area on the surface with poor image quality.
It is known that a surface may be microscopically observed with a technique known as dark field illumination. This technique involves illuminating a sample with light having an angle of incidence in one range and observing the light at an angle of reflection in a different range so that light that is specularly reflected from the surface of the sample is never directly observed. Thus, the field of view is always "dark" except for light that is diffusely reflected by the sample into the field of view. Dark field illumination therefore does not contemplate simultaneously illuminating a surface at grazing angles of incidence and observing specularly reflected light.